Abstract: The characteristics of natural circulation temperature fluctuation were studied experimentally and theoretically. Experimental results show that rolling motion may cause the coolant fluctuation which results in the periodical oscillation of wall and fluid temperatures. The amplitude of wall temperature fluctuation is larger than that of fluid temperature oscillation. Because of the wall heat capacity, there is a phase difference between the fluctuation curves of fluid and wall temperatures. A mathematical model was also conducted to simulate the temperature fluctuation under coolant flow fluctuation condition. The theoretical results agree very well with experimental data.